Shaped cathode for electrolytic etching



June 1%? J. J. FRANTZEN 3,32 3

SHAPED GATHODE FOR ELECTROLYTIC ETCHING Filed Nov. 13, 1963 INVENTORJOHN J. FHA/V7125 A! ATTQRNEY United States Patent 3,325,384 SHAPEDCATHODE FOR ELECTROLYTIC ETCI-IING John J. Frantzen, St. Paul, Minn.,assignor to Buckbee- Mears Company, St. Paul, Minn., a corporation ofMinnesota Filed Nov. 13, 1963, Ser. No. 323,476 3 Claims. (Cl. 204-143)This invention relates to electrolytic etching of circuit patterns inmetallic workpieces and in particular is directed towards means forensuring properly etched circuits of a variety of configurations.

In electrolytic etching a suitable source of DC. energy is connectedacross the anode, the workpiece, and the cathode with the latter twoimmersed in a suitable liquid electrolyte. The pattern to be etched isdefined by a mask of etch resist material on the workpiece. The workfaceof the cathode is positioned with respect to the workpiece such thatupon activating the energy source electrical current flow between thecathode and the workpiece removes or etches away those areas of themetallic workpiece not coated with resist material. The etching rate isdeepndent upon many factors, one of which is the displacement betweenthe cathode workface and the corresponding area of the workpiece.Obviously the greater the displacement the slower will be the rate ofetch.

In the past there has been a limitation on the circuit designs which canbe fabricated by electrolytic etching. Recognizing that there must beelectrical circuit continuity from the energy source to the portion ofthe workpiece being etched through the electrolyte and the cathode backto, the energy source, circuit designs which could result indiscontinuities in this electrical circuitry during etching heretoforecould not be electrolytically etched. For example, consider the case ofetching out a plurality of radially displaced annular areas from a flatmetallic workpiece. Assuming that the energy source is connected to aperipheral area of the metallic workpiece, it was very possible that anouter annular area would be etched sufficiently to break electricalcircuit continuity to an inner area before the latter was completelyetched. Etching of the latter would then terminate, resulting in anincomplete article. Rather than take a change of this occurring, circuitpatterns of this type were generally obtained by some other lessdesirable process.

The principal object of this invention is to provide means to insurethat electrolytic etching progresses in a predetermined manner and rateso as to enlarge the range of circuit pattern designs which can beelectrolytically etched by overcoming the disadvantages described above.This principal object is achieved by providing a cathode workface whichis contoured to control the etching process. For example, the cathodeworkface surface may be tapered to provide different displacementsbetween the cathode and the workpiece thereby resulting in a differencein the rate of etch of diiferent sections of the workpiece.

These and other objects and features of this invention will becomeapparent during the course of the following detailed description withreference to the accompanying drawings in which:

FIG. 1 illustrates an embodiment of this invention incorporated in anelectrolytic etching apparatus for etching out of the workpiece acircuit pattern of concentric rings;

FIG. 2 is a sectional view of a part of the apparatus of FIG. 1.

In the figures the metallic workpiece comprises a thin layer of gold 11bonded with a suitable insulative adhesive 12 to a supporting base of asheet of aluminum ice 13. The concentric rings on the top surface of theworkpiece illustrate the pattern to be etched in the work-. piece andare defined by masking the workpiece with etchresist material 14. Theunshaded areas in FIG. 1 indicate those exposed areas from which thetold is to be etched away while the shaded areas are protected duringetching by the resist. Briefly, the workpiece can be prepared in thefollowing manner. The gold is initially vapor blasted to mat its finishand then cleaned with a suitable solvent. A proper thickness of aluminumsheeting is cut into rectangular pieces which are cleaned and smoothenedand then covered with a suitable adhesive material. The gold is thenplaced on the adhesive and its multiple layer assembly is subjected to asuitable pressure for a predetermined period of time to bond the layerstogether. To photoprint the desired circuit pattern on the workpiece,the gold is first thoroughly cleansed by light scrubbing, rinsing andthen drying. Then the gold is coated with a suitable light sensitivematerial or enamel upon which is photoprinted the desired pattern in thewell-known mane ner. Where the gold is protected, the light sensitivematerial is baked on, and in those areas where the gold is to be removedduring etching, the enamel is removed during the photoprinting processso the selective areas are exposed for etching. It should be understoodthat the foregoing is only briefly illustrative of a process forpreparing the workpiece and this and other methods are well known tothose of ordinary skill in the art and can be used without detractingfrom the essence of the invention.

At a point on the peripheral area of the workpiece 10, an outputterminal of the DC. energy source 15 is connected to the workpiece by asuitable electrical conductor 16. The cathode 17 is also connected toanother output terminal of the energy source by a further conductor .18.The electrical circuit path is closed by a suitable electrolyte, notshown, in which the workpiece and the cathode are immersed. Thissolution may be a mixture of nickel chloride and reagent HCl. When theenergy source 15 is activated, the electrical current passing betweenthe workpiece and the cathode causes removal of the gold from the topsurface of the workpiece in those areas not protected by the coveringcoating of resist material 14.

The workface of the electrode 17 is conical shaped and the remainder iscylindrical so that overall it somewhat resembles a sharpened stubbypencil. The apex of the cone is centered with respect to the annularareas to be etched out of the workpiece and is spaced a short distanceabove the workpiece. A flat projection of the tapered surface of thecathode would substantially encompass the extremities of the areas to beetched out of the workpiece. It was previously pointed out that theetching rate is dependent, inter alia, upon the displacement between theworkpiece and the workface of the cathode. It can be visualized thenthat with the arrangement illustrated in FIGS. 1 and 2 the etchingcurrent will be concentrated to a greater degree at the center area ofthe workpiece than it is toward the outer peripheral areas thereof. Theetching rate will correspondingly be faster at the center than at theperiphery. In this manner then the annular areas will be etched out insuccessive order starting at the center of the workpiece radiallyoutward so that the electrical circuit continuity between he energysource 15 and all areas of the workpiece will be maintained until therespective annular areas have been completely etched out.

In the practice of this invention, the workpiece 10' is prepared in themanner previously described or by some other equally suitable method.The workpiece and the cathode 17 are then detachably mounted in aholding fixture 20 which maintains orientation and position of the twounits with respect to one another such as illustrated in FIGS. 1 and 2with the apex of the cathode centered above the workpiece. Withconductors 16 and 18 connecting energy source to the workpiece andcathode respectively, the so-assembled units are immersed in theelectrolyte and the energy source is activated. Although the amount ofetching current and the period of time of application is a matter ofchoice and readily determinable either empirically or otherwise by thoseof ordinary skill in the art, in a typical case the apex of the cathodecone may be of an inch above the center of the workpiece and while thecombination is immersed in the electrolyte of nickel chloride andreagent HCl alternately, 15 amps. are applied for ten seconds and 4amps. for four minutes until the etching of the annular areas iscomplete, which may take in the order of twenty minutes.

After the etching is complete and the workpiece is detached from theholding fixture, it is cleaned by scrubbing with Triad, is then pumicedand dried in air. If desirable, the etched-out .annular areas can befilled with an insulating epoxy flush with the surface of the gold.

Although only one particular embodiment of this invention has beendescribed, viz., a cone-shaped cathode in combination with a flatworkpiece to etch out annular areas, obviously the teachings of theinvention can be incorporated with the resulting advantages and featuresinto apparatus for producing a variety of etched-out patterns. For anyparticular pattern to be etched out, the corresponding configuration ofthe workface of the cathode can be determined by design or empiricallyto provide a rate and manner of etching which will maintain theelectrical circuitry continuity until the etching of all thedisconnected areas is complete.

I claim:

1. A process for electrolytically etching a thin flat metallic workpiecemasked to form a plurality of separate concentric annular areas out ofthe workpiece comprising: electrically connecting a source of energy toan outer edge of said workpiece; supporting a cathode having aconeshaped workface spaced away from said workpiece with the apex of thecone centrally located with respect to said workpiece such that energyconcentration between the workpiece and the cathode is greater at thecenter than it is at the periphery of the workpiece during etching andprogressively electrolytically etching said workpiece outwardly from thecenter to form said annular areas.

2. A process for electrolytically etching a plurality of radiallydisplaced separate, concentric, annular areas from a thin fiat metallicworkpiece having masked annular areas thereon comprising: a positioningpointed cylindrical cat11- ode orthogonal to said workpiece with thepoint spaced away from but centrally located with respect to theworkpiece, the tapered area of the cathode between the point and thecylindrical section being substantially equal to the area of theworkpiece in fiat projection; and etching the unmasked areas betweensaid respective masked annular areas completely through progressivelyfrom the center radially outward.

3. A process for electrolytically etching out of a masked thin flatmetallic workpiece a group of disconnected areas comprising the stepsof: electrically connecting a source of energy to an outer edge of themasked workpiece; placing a cathode having a uniform. upwardly taperedworkface over the masked workpiece so that the lowest part of thecathode workface is located furthest from the edge of the workpiece towhich the energy source is connected and the tapered area is superim-.

posed over the workpiece; and keeping the workpiece and cathode in thesame relative positions while etching the workpiece progressivelyoutwardly from the area located directly under the lowest part of thecathode.

References Cited UNITED STATES PATENTS 2,492,214 12/1949 Fonda 2042242,844,531 7/ 8 Prince 204-224 3,05 8,895 10/ 1962 Williams 204-3343,239,441 3/1966 Marosi 204l43 FOREIGN PATENTS 809,936 3/ 1959 GreatBritain.

JOHN H. MACK, Primary Examiner.

D. R. JORDAN, Assistant Examiner.

1. A PROCESS FOR ELECTROLYTICALLY ETCHING A THIN FLAT METALLIC WORKPIECEMASKED TO FORM A PLURALITY OF SEPARATE CONCENTRIC ANNULAR AREAS OUT OFTHE WORKPIECE COMPRISING: ELECTRICALLY CONNECTING A SOURCE OF ENERGY TOAN OUTER EDGE OF SAID WORKPIECE; SUPPORTING A CATHODE HAVING THECONESHAPED WORKFACE SPACED AWAY FROM SAID WORKPIECE WITH THE APEX OF THECONE CENTRALLY LOCATED WITH RESPECT TO SAID WORKPIECE SUCH THAT ENERGYCONCENTRATION BETWEEN THE WORKPIECE AND THE CATHODE IS GREATER AT THECENTER THAN IT IS AT THE PERIPHERY OF THE WORKPIECE DURING ETCHING ANDPROGRESSIVELY ELECTROLYTICALLY ETCHING SAID WORKPIECE OUTWARDLY FROM THECENTER TO FORM SAID ANNULAR AREAS.